Cascaded multilevel converters (CMC) are widely used in electrical energy conversion, high voltage frequency conversion drive, high-voltage direct current transmission, reactive power compensation and other applications. Normally, a cascaded multilevel converter includes a single-phase switch bridge or half-bridge inverter circuits which are connected in series. An integrate insulation resistance of the cascaded multilevel converter should be large enough to avoid a safety risk caused by an overlarge leakage current due to a small integrate insulation resistance during the use of the cascaded multilevel converter.
In practice, leakage protection and insulation resistance detection need to be performed on the cascaded multilevel converter by a dedicated circuit. This need is especially prominent for a photovoltaic power generating system. For example, it is required in a safety regulation that a cascaded multilevel converter should not be started until its insulation resistance reaches a certain level. At present, a conventional insulation resistance detection solution is only adaptive for calculating an insulation resistance value of a single converting circuit or a single inverter, and cannot be used to measure and calculate an insulation resistance value of a cascaded multilevel converter.